the presence of the Duchesss of Youk ( Her Royal Highness, don't forget ) tended to make the shopkepers of Bath push up their prices to suit her presumably long pockets.

Also if she were at Bath, there would be others who would come to see her , social climbers and tourists. There would be an influx of visitors and so again the shopkeepers wold try to capitalise on the situation. Simple economics ;-)

A bit about them. Famed ( or defamed ,rather-see below)as the 'The Grand Old Duke of York' Frederick was the second son of George III. He was Commander-in-Chief of the British Army, between 1798 and 1809. Unsuccessful in the field during the French Revolutionary and Napoleonic Wars, he is better remembered for putting down corruption in the administration of the army. However, in 1809, he was dismissed from office after revelations that he and his mistress Mary Anne Clarke had been selling army commissions.Hence teh nursey rhyme,The Grand Old Duke of York:

HIs conduct as commander-in-chief had considerable influence on the history of the British army. He supported the commanders' efforts to revive military spirit with some success; he looked after the soldiers and their comforts, and sternly put down the influence of personal favouritism. Despite his involvement in the Mary Anne Clarke scandal, he did much to eradicate political jobbery in military appointments, and systematic corruption. The irony has been noted that, despite his contribution to the nation's defence and his improvements to the army, ‘the Duke is now chiefly remembered in the public mind as a man who marched his army up and down a hill and ran it as a commercial proposition, with the aid of his mistress’
( See Roger Fulford Royal dukes: the father and uncles of Queen Victoria, edn (1973)

HIs Duchess was Princess Frederica of Prussia (1767–1820), eldest daughter of Frederick William II, king of Prussia, and his first wife, Elizabeth of Brunswick. The couple had met in Berlin and the marriage was celebrated there on 29 September 1791, and at Buckingham House (later Palace) on 23 November. There were no children of the marriage; husband and wife soon separated, and the duchess of York retired to Oatlands Park, near Weybridge, Surrey. A modest, self-effacing woman, she cared little for London society, preferring the company of her pet dogs, and attending church every Sunday. Her sympathy with the estranged princess of Wales and her daughter Charlotte incurred the prince regent's resentment, but she was liked and respected by the rest of the family. She died on 6 August 1820, of consumption, and was buried in Weybridge church on 14 August.

Presumably,she was taking thw waters at Bath in an attempt to become pregnant.For many women that was the only thing the medical science of the day could do offer them as a cure for infertility.

Poor Mrs Lloyd is the one who likes fish. JA would like her and Martha to retire to Bath too, so she is joking with Cassandra- something along the lines of if we want them to come here , DON"T tell Mrs Lloyd hhow expensive it is !